(a) Field of the Invention
The present invention relates to a photosensitive resin composition for use in the preparation of a printed-wiring board, and more specifically, to a solventless liquid photosensitive resin composition that can be developed and stripped off using an aqueous alkali solution.
(b) Description of the Prior Art
A printed-wiring board is normally prepared by forming a photosensitive layer on a base board. This is typically done in one of two ways. According to the first method, a solvent-containing photosensitive composition, i.e., a photoresist, is coated on the copper surface of a copper-foil laminate (hereinafter referred to as a base board for printed circuits) and thereafter dried to thereby form a photosensitive coated film. Alternatively, the board can be prepared by applying, i.e., laminating, a dry film (that comprises applying a dry photoresist film, as a photosensitive film, between a cover film made of polyethylene or the like and a base film made of polyester or the like. In this case, the cover film is stripped off, and the exposed photosensitive film together with the base film are adhered to the copper surface. Once the photosensitive film is present on the board, according to either method of application, a negative film having a desired circuit pattern is contacted with the film (in the case of the dry film, it is contacted with the surface of the base film). The film is then exposed under vacuum to thereby cure the coated film or dry film on the exposed portion. Thereafter, the noncured coated film or dry film is removed by means of a developer, for instance an aqueous solution of alkali such as sodium carbonate. The thus exposed copper portion is then etched and removed by means of an etching liquid that is normally comprised of an aqueous solution of ferric chloride or cupric chloride. The remaining coated film or dry film, namely the protective film, is then stripped off by means of a stripping liquid, for instance an aqueous solution of alkali such as sodium hydroxide.
The photoresist or dry film may alternatively be used as a plating resist. In this instance, the steps prior to the developing treatment are carried out in the same manner as described above to thereby form a pattern. The exposed copper portion is subjected to copper sulfate plating, soldering or the like, and the plating resist is then stripped off. Thereafter, etching is effected.
Accordingly, the photoresist and dry film for use in the printed-wiring board prepared using the above-mentioned photoengraving technique advantageously should have superior performance characteristics, such as coating ability (laminating ability in the case of the dry film), uniformity of the coated film, curing (sensitivity) and developing ability, further adhesion and hardness of the cured coated film, stripping ability of the cured coated film, resistance to etching liquids and various platings, and the like. However, use of the above photoresist has encountered various problems which need to be solved. For example, because the photoresist is of a solvent type which requires dissolving a photosensitive material in a solvent, it necessitates a drying step to volatilize the used solvent after a coated film has been formed, thereby making the continuity of production line worse. Additionally, because the photoresist contains the solvent, the viscosity of the photoresist is apt to change when coating, thereby making the control of film thickness and the formation of uniform coated films difficult. Also, the organic solvent contained in the photoresist exerts a bad influence upon the human body, and so must be placed under control.
In view of this, there has recently been observed a growing tendency to replace the photoresist with the dry film. However, the dry film is defective in that it is expensive and further makes the continuity of production line worse in comparison to that of the photoresist because it needs a holding time of about 15 minutes in order to stabilize a coated film after lamination and exposure have been completed. For these reasons, there has recently been proposed a process of using a solventless liquid photoresist. This process comprises coating the liquid photoresist on the copper surface of a base board, then placing a negative film in an off-contact state directly on the resulting coated film without a drying or holding step, and then radiating parallel light thereon.
As an example of a solventless liquid photoresist used in this process, U.S. Pat. No. 4,436,806 (Japanese Laid Open Patent Application No. 164595/1982) describes a photoresist consisting essentially of a polyene-thiol compound. However, this solventless liquid photoresist is not preferable because it requires complicated steps for manufacturing the printed-wiring board, such as preliminary UV curing to exposure, UV post-curing after development, and the like. In addition, the known photoresist smells bad because it contains thiol and the like. Furthermore, when it is used as plating resist, a chelate compound is formed by reaction of copper with thiol to thereby make plating adhesion worse. In any case, the fact is that there has hitherto been unknown a solventless liquid photoresist which can concurrently satisfy the performances required for the printed-wiring board, namely (1) the rate of polymerization at the time of exposure, which is related to the curing ability of coated films, (2) the viscosity and leveling ability of liquid, which properties are related to the coating ability, and uniformity and developing ability of coated films, (3) the hardness, adhesion and stripping ability of cured films, and resistance to etching liquid and various plating liquids, and further (4) superior adhesion of plating to the exposed copper portion.